1.x-style dirty checking has been reinstated for better performance. An optional mixin is
available for elements to skip dirty checking of objects and arrays, which may be more easy to
integrate with some state management systems. For details, see
Using the MutableData mixin in Data system concepts.

Support for dynamically-created custom-style elements has been added.

Support for the external style sheet syntax, <link rel="import" type="css"> has
been added. This was deprecated in 1.x, but will be retained until an alternate solution is
available for importing unprocessed CSS.

New properties rootPath and basePath were added to Polymer.Element to allow authors
to configure how URLs are rewritten inside templates. For details, see the
Update URLs in templates in the Upgrade guide.